Could it be done? Yes. Is it worth it? Probably not. The only way I could think of doing that would be to use a dll that intercepts anything mIRC sends to the server, search for a QUIT command being sent, then changing this command to do what you wanted. It wouldn't be to easy to do though. You'd need to either use a library (such as MS Detours) or write your own code (very difficult) to override the send() function in WinSock. In addition you'd need to be familiar with how socket programming in C/C++ works and how the send() function works so that you can filter text. IMHO it's more work than it would really be worth.
